Table of Contents
Abstract
Objective
Chapter1. Introduction
1.1) OFDM versus FDM
1.2) Pros and Cons of OFDM system Chapter
2.OFDM System Description
2.1) Block Diagram
2.2) Channel Coding and Decoding
2.3)Interleaving and Channel Diversity
2.4) Digital Modulation/Demodulation
2.5) Serial to parallel/Parallel to seria
2.6) Parallel to Serial Conversion
2.7) Generation the subcarriers using IFFT
2.8) Guard time and Cyclic Prefix
2.9) Synchronization and Channel Estimation
2.10) Pulse shaping and match filter Chapter 3: Implementation
3.1) Transmitter Implementation
3.2) Receiver Implementation
3.2.1) Frequency Offset Estimation:
3.2.2) Synchronization
3.2.3) Channel Estimation and Equalizer
3.3.4) Channel decoder
3.3.
4.1) Convolution (Viterbi decoder)
3.3.
4.2) Turbo decoder
3.3.
4.3) LDPC decoder Chapter 4: Results
4.1) Simulation Result
4.2) USRP
4.3) Presentation
4.4) GUI
Conclusion
Reference
Abstract:
Communication systems with multiple carrier frequencies, such as systems based on orthogonal frequency division multiplexing (OFDM), have recently become increasingly popular. The goal of this project is to send and receive data over a real wireless link using analog and digital hardware based on the USRP2 with xcvr2450 daughterboard’s. While signals are transmitted over the air in a realistic fashion the processing will be done off-line in Matlab. Thus the data sequences to be transmitted are created in Matlab, downloaded to the hardware, and sent on the channel (at the actual data-rate) of the transmitting node. The signal is captured on a receiving node, and saved for post-processing.
At the receiver, Synchronization, Frequency offset estimation, channel (estimation and) equalizer and channel decoders are done. Proper Synchronization, Frequency offset estimation and channel estimation and channel equalizer is crucial in OFDM system to preserve orthogonality between subcarriers. Convolutional code is recommended in IEEE802.11, which is implemented besides, advanced coding methods like Turbo and LDPC can improve the system performance. In addition, Turbo and LDPC code are tested on USRP system and the results are shown in the last chapter of this report.
A graphically user interface (GUI) is designed in Matlab which make system easier to understand and apply and view in a quite well-fashion. In this project, quite a well-practical method is devised for SNR and BER estimation.
Finally, performance evaluation results in simulation and comparing with real wireless system, gives outstanding perception of real frequency-selective fading channel behavior.
CHAPTER 1
Introduction
Recently, a worldwide convergence has occurred for the use of Orthogonal Frequency Division Multiplexing (OFDM) as an emerging technology for high data rates. In particular, many wireless standards (Wi-Max, IEEE802.11a, LTE, and DVB) have adopted the OFDM technology as a mean to increase dramatically future wireless Communications. OFDM is a particular form of Multi-carrier transmission and is suited for frequency selective Channels and high data rates. This technique transforms a frequency-selective wide-band channel into a group of non-selective narrowband channels, which makes it robust against large delay spreads by preserving orthogonality in the frequency domain. Moreover, the ingenious introduction of cyclic redundancy at the transmitter reduces the complexity to only FFT processing and one tap scalar equalization at the receiver [1]. With the increasing demand for high data rate transmission, different methods are applied in communication systems to combat frequency- selective fading channel effect. Since when data is transmitted at the rate of several mega bit per second, then the delay time of delayed signal is greater than one symbol duration. Conventional receivers, try to equalize signal using for example adaptive equalizer. However there are practical difficulties in operating this equalization at several megabits per second with compact, low-cost hardware [2]. OFDM is a method which divide bandwidth to subcarriers which each subcarrier experiences flat fading. So OFDM system by applying FFT, make equalization straightforward.
1.1)OFDM versus FDM:
In Frequency Division Multiplexing (FDM) systems, total bandwidth is divided to N non-overlapping frequency sub-channels. In other words, in order not to have inter-carrier interference, sub-channels are separated in frequency domain. Besides, between sub-channels there is guard interval to avoid side-lobes interference as well. As it can be seen, the FDM method is not efficient in spectrum usage. OFDM system solves this problem by dividing bandwidth to N overlapping subcarriers which still preserve orthogonality. This method in vented in 1960 but hasn’t used in industry till appearance of high speed DSPs implementing and Fast Fourier Transform (FFT) on them. The following picture shows difference between FDM and OFDM explicitly.
Figure.1: Efficient usage of spectrum in OFDM system(b) in compare to FDM system(a)
1.2)Pros and Cons of OFDM:
The OFDM transmission scheme has the following key advantages:
? OFDM is an efficient way to deal with multipath; for a given delay spread, the implementation complexity is significantly lower than that of a single-carrier system with an equalizer.
? In relatively slow time-varying channels, it is possible to enhance capacity significantly by adapting the data rate per SC according to the signal-to-noise ratio (SNR) of that particular SC.
? OFDM is robust against narrowband interference because such interference affects only a small percentage of the SCs.
? OFDM makes single-frequency networks possible, which is especially attractive for broadcasting applications.
On the other hand, OFDM also has some drawbacks compared with single-carrier modulation: ? OFDM is more sensitive to frequency offset and phase noise.
? OFDM has a relatively large peak-to-average-power ratio, which tends to reduce the power efficiency of the radio frequency (RF) amplifier [1].
Since in this project, Physical Layer of OFDM system based on IEEE802.11 is implemented so here, the main part of this physical Layer is mentioned.
Chapter2
OFDM System Description
In the single carrier transmission, if we use one of the M?PSK or M?QAM modulation techniques as an example, the data rate will be almost R=log2(M)T?1s, but there is channel delay limitation for this rate, to avoid this limitation, trend going to divide this rate over K carriers with smaller rates, and this techniques does not affect the total bandwidth B as the bandwidth for each subcarrier equal B/K, and higher data rate K times larger for the same channel delay spread.
The symbol duration increase by increasing K and this limited K, as the symbol duration become more sensitive to selective channel, in the IEEE standard for the OFDM systems, 52 subcarriers used in total, 48 for data and 4 as pilot carrier.
The implementation for the parallel carriers occurs in two criteria, the first criterion to modulate each carrier independently in the time domain, and the other to pass stream of data over K band pass filters excited by parallel data, modulation is the frequency domain, the last criterion is the most used in the practical situations and the same as mentioned in the IEEE standard.
The baseband signal can be represented as s t=e j2πf k t s kl g t?lT s
k where s kl represent
l
complex modulated signal by PSK or QAM, Ts is the symbol duration, g is the pulse shape and the index l can be for the time domain between –inf to inf, or for the frequency domain between 0 to k?1, as illustrating in figure 1 below.
Figure 2: Spreading series data over multicarrier system
The block diagram for a general OFDM system is described as
Figure 3: OFDM Block Diagram
2.1 Block diagram
The OFDM transmitter and receiver model that we used in our project has following block diagram.
Figure 4: OFDM system for our Project
Now each component of the block diagram will be explained individually.
2.2 Channel Coding
Channel encoding inserts additional information into a transmitted bit stream to facilitate error detection and error correction at the receiver. Block coding breaks up a bit stream into words of length k bits and appends check bits to form a codeword of length n bits. A corresponding channel decoder examines the complete codeword, and detects and even corrects certain types of erroneous bits caused by the channel.
In this project, three types of channel encoder / decoder were used. First was the Convolutional Encoder / Decoder, second was Turbo encoder / Iterative BCJR Decoder and the third one was low-density parity-check (LDPC) encoder and decoder.
2.2.1 Convolutional Coding
We used Convolution Encoder and the Corresponding Viterbi Decoder for our system according to IEEE 802.11a specifications. The Encoder Uses industry-standard generator polynomials g0=(133)8 and g1=(171)8, of rate R=1/2, as shown in Figure 1.
Figure 5: Convolutional Encoder
In accordance with the IEEE 802.11a specifications, The DATA field composed of SERVICE, PSDU, tail and pad bits shall be coded with the convolution encoder of rate R=1/2, 2/3 or 3/4 corresponding to the desired data rate. Higher code rates are employed from the half rate encoder by using puncturing. Puncturing is a process of omitting some of the coded bits before transmission (Thus reduces the number of bits to be transmitted). Bits are omitted using a predefined puncturing matrix in the transmitter, and receiver uses the ‘zero’ metric in omitted bit positions in the decoding process.
We used following puncturing matrices for our system.
‘1’ in the matrix indicates the transmitted bit and ‘0’ indicates that bit is removed. For example if we want to make 2/3 code rate using appropriate matrix from above table, We will take basic encoder output and will transmit every bit from the first branch (output data A in Figure 1) and every second
bit from the second branch (output data B in Figure 1). Figure 2 shows the omitted bit positions for 2/3 rate coding using above puncturing matrix.
An Example of Bit stealing (R=2/3)
Then Divide the encoded bit string into groups of NCBPS bits. Within each group, perform an “interleaving” (reordering) of the bits according to a rule corresponding to the desired RATE.
2.2.2 Turbo Codes
A turbo code is formed by parallel computation of two codes separated by an interleaver. The generic structure of the Turbo Codes is as shown in Figure 4.
Figure 6: Generic Turbo Encoder
The two encoders are normally identical. The code is systematic and the task of the interleaver is to scramble bits in a pseudo random fashion. Thus by using the interleaver the outputs of the two encoder are totally different. So if one encoder outputs a low weight codeword there is less probability that the other encoder will output the low weight codeword too. So there is smaller chance of producing output with very low weight codeword. Higher weight is beneficial for the performance of the decoder.
In this project the UMTS turbo encoder is used which is shown in Figure 5.
Figure 7:Turbo Encoder used in this project
The encoder is 1/3 rate with two Recursive Convolutional encoders connected in parallel via an interleaver. A pseudo random interleaver is used in this project. There is no such thing as a universally best interleaver, for short length block sizes odd-even interleaver has been found to outperform the pseudo random interleaver. The best choice of the interleaver is dependent on the code design. The output of the encoder is a systematic bit and 2 parity bits. We implemented ? rate turbo codes by puncturing. The puncturing is done by alternatively taking the parity bits for each step. So output of the ? rate encoder at any time will be a systematic bit and one of the parity bits of 1/3 rate encoder.
2.2.3 LDPC Channel Encoder:
Channel encoding inserts additional information into a transmitted bitstream to facilitate error detection and error correction at the receiver. Block coding breaks up a bitstream into words of length k bits and appends check bits to form a codeword of length n bits. A corresponding channel decoder examines the complete codeword, and detects and even corrects certain types of erroneous bits caused by the channel.
In this project, low-density parity-check (LDPC) encoder and decoder was one of the encoder used. LDPC code is a linear error correcting code, a method of transmitting a message over a noisy transmission channel, and is constructed using a sparse parity check matrix. The name comes from the characteristic of their parity-check Matrix which contains only a few 1’s in comparison to the amount of 0’s.Their main advantage is that they provide a performance which Is very close to the capacity for a lot of di?erent channels and linear Time complex algor ithms for decoding.
Several di?erent algorithms exist to construct suitable LDPC codes. Gallagher (LDPC inventor) himself introduced one. Furthermore MacKay proposed one to semi-randomly generate sparse parity check matrices. This is quite Interesting since it indicates that constructing good performing LDPC Codes are not a hard problem. In fact, completely randomly chosen Codes are good with a high probability. The problem that will arise, Is that the encoding complexity of such codes is usually rather high.
The following tanner graph, figure 3, for the LDPC showing circles in the Tanner graph represent the bits that are transmitted (“variable node”). The squares with a “+” inside of them represent the parity check constraints (“check node”). Where each variable node has degree dv and each check node has degree dc.
Figure 8: Tanner graph for LDPC matrix
2.3 Interleaving and Channel Diversity
To introduce the diversity in the radio transmission, and to achieve full diversity gain of the code, transmitted bits must introduce to different uncorrelated fading channels, this can be achieved in two methods:
1-time interleaving used in single carrier systems, separation between closely related bits used to achieve the time interleaving and cost coding delay to the system.
2-frequency interleaving in the multicarrier systems, the situation is different in the multicarrier systems than the single carrier as the time and frequency correlation are independent to each other, the correlation in time has its own time variance due to Doppler spread caused by multipath
channel, t corr=c
fv , where c, f and v are the velocity of light, carrier frequency and the relative speed
consequently.
While the correlation in frequency has its own frequency selective parameters due to different travelling time caused by my multipath channel, f corr=τm?1, where τm is the delay spread of the channel.
In this project based on the IEEE standard, time interleaving in the multicarrier system used over blocks, where the data pass through pseudorandom permutation in the transmitter side and the inverse in the receiver side.
Channel Decoding will be defined in detail in the next chapter.
2.4 Digital Modulation/Demodulation:
Achieving high data rates and low error rates are vital in wireless communications. High data rates can be achieved using higher-order modulations (e.g.,
M?ary QAM,M?ary PSK). Low error rates can be
achieved using packet retransmissions.
BPSK:mapping binary bits to symbols based on the
following map table in the transmitter side and reverses it in
2
to
normalize the constellation power.
-QPSK: mapping binary block of two bits to symbols based on the following map table in the transmitter side and reverses it in the receiver side, [00-0; 01-1; 10-2; 11-3] .with scaling factor to normalize the constellation power.
equal
2
-16PSK: mapping binary block of four bits to symbols based on
the following map table for the Inphase and the Quadrature
phase components, in the transmitter side and reverses it in the
receiver side, [00-0; 01-1; 10-2; 11-3] .with scaling factor equal
10
to normalize the constellation power.
-64PSK: mapping binary block of six bits to symbols based on the
following map table for the Inphase and the Quadrature phase components, in the transmitter side and reverses it in the receiver side, [000-0; 001-1; 010-2; 011-3; 100-4; 101-5; 110-6; 111-7] .with scaling factor equal
to normalize the
42
2.5 Serial to parallel/Parallel to serial:
In an OFDM system, each channel can be divided into various sub-carriers (64 in this project). The use of sub-carriers makes optimal use out of the frequency spectrum but also requires additional processing by the transmitter and receiver. This additional processing is necessary to convert a serial bitstream into several parallel bitstreams to be divided among the individual carriers (serial to parallel converter). Once the bitstream has been divided among the individual sub-carriers, each sub-carrier is modulated as if it was an individual channel before all channels are combined back together and transmitted as a whole. The receiver performs the reverse process (parallel to serial converter) to divide the incoming signal into appropriate sub-carriers and then demodulating these individually before reconstructing the original bitstream.
2.6 Parallel to Serial Conversion:
Once the cyclic prefix has been added to the sub-carrier channels, they must be transmitted as one signal. Thus, the parallel to serial conversion stage is the process of summing all sub-carriers and combining them into one signal. As a result, all sub-carriers are generated perfectly simultaneously.
2.7 Generation the subcarriers using IFFT:
An OFDM signal consists of a sum of subcarriers which are modulated by using Phase Shift Keying (PSK) or Quadrature Amplitude Modulated (QAM).
In OFDM system all sub carriers have the same
amplitude and phase angel, but in practice the
amplitudes and phases may be modulated differently
for each sub carrier.
In practice, parallel data modulation and coherent demodulation can be simply done by using inverse fast Fourier transfer (IFFT) and fast Fourier transfer (FFT).
As an example of how to generate an OFDM symbol, let us assume that we want to transmit eight binary values [1 1 1 -1 1 1 -1 1] on eight subcarriers, the
IDFT or IFFT then has to calculate as shown in the matrix
equation below.
The left hand side contains the IDFT matrix, where every
column correspond to a complex subcarrier with
normalized frequency ranging from -4 to 3, the right hand
side gives the eight IFFT output samples that form one
OFDM symbol.
The first half of the rows correspond to positive
frequencies, while the last half correspond to negative
frequencies, hence, if oversampling is used, the zeros should be added in the middle of the data vector than appending them at the end. This ensure the zero data values are mapped onto frequencies close to plus and minus half the sampling rate, while the non zero data values are mapped onto the subcarriers around 0 Hz, for the previous example, the oversampled input vector would become [1 1 1 -1 0 0 0 0 0 0 0 0 1 1 -1 1]
1 8
11
1
1
2(1+j)
11
j
1
2(?1+j)
1j
1
1
2(?1+j)
?1?j
?j
1
2(1+j)
11
?1
1
2(?1?j)
11
?j
1
2(1?j)
1j
?1
1
2(1?j)
?1?j
j
1
2(?1?j)
1?1
1
1
2
2(?1?j)
1?1
j
1
2
2(1?j)
1?j
1
1
2(1?j)
?1j
?j
1
2(?1?j)
1?1
?1
1
2
2(1+j)
1?1
?j
1
2
2(?1+j)
1?j
?1
1
2(?1+j)
?1j
j
1
2(1+j)
1
1
1
?1
1
1
?1
1
=
1
8
4
2(1+j 2?1)
2+2j
? 2(1+j 2+1)
? 2(1+j 2+1)
2?2j
2(1?j 2?1)
2.8 Guard time and Cyclic Prefix:
The guard time is chosen larger than the expected delay spread, such that multipath components from one symbol cannot interfere with the next symbol. The guard time could consist of no signal at
all. However, in that case the problem of inter carrier interference (ICI) would arise. ICI is cross-talk between different subcarriers, which means they are no longer orthogonal and the effect of multipath with zeros signal in the guard time; the delayed in a subcarrier causes inter carrier interference (ICI) on other subcarrier and vice-versa as illustrated in figure 5 between subcarrier one and two.
To eliminate inter carrier interference (ICI), the OFDM symbol is cyclically extended in the guard time. This ensures that delayed replicas of the OFDM symbol always have an integer number of cycles within the FFT interval, as long as the delay is smaller than the guard time. As a result, multipath signals with delays smaller than the guard time cannot cause ICI.
) Cyclic prefix causes loss of a portion of signal energy, given by: SNR CP_loss=?10log?(1?T CP
T symbol
Figure 9: ICI between two subcarriers because of multipath effect.
2.9 Synchronization and Channel Estimation:
In the case of the miss synchronization, the original signal of OFDM and the delayed version are no more orthogonal, and this yield to ISI in the frequency and time domain, updating the transmitted signal with Cyclic prefix reserve the orthogonality in the present of multipath channel, this cyclic prefix will add guard interval to OFDM symbol Δ, and as long as the symbol delay τhold this identity, τ<Δ, the orthogonality between the transmitter and the receiver pulse still hold, the received pulse rotated by phased factor must be calculated in coherent modulation.
OFDM System split the single carrier high rate data into higher carriers with low data rate, this make the system more robust against multipath, and more robust against time synchronization using the guard intervalΔ.
In the IEEE standard, OFDM symbols of length 2?Ts is used to synchronize and estimation of channel coefficients c kl.
Accurate frequency synchronization is important issue in OFDM systems, frequency tracking mechanism applied when the frequency measurement δf become available, the effects if residual frequency offset presents has two effects:
1-Corrupted orthogonality between the transmitter and the receiver pulses.
2-Time variant phase rotation in the received pulse.
The frequency shift that is given by the multiplication with exp(j2πδft)means that the modulated symbols are rotated by a phase angle 2πδf.
Based on the IEEE standard, extra carriers called continuous pilots used for frequency synchronization and to calculate the Doppler bandwidth.
2.9.1 Synchronization using special training symbols:
For high rate packet transmission, the synchronization time need to be as short as possible, preferable a few OFDM symbols only. To achieve this, special OFDM training symbols can be used for which the data content is known to the receiver (preamble), in this way, the entire received training sequence can be used to achieve synchronization.
A long OFDM training symbol based on the IEEE standard consists of 53 subcarriers (including a zero value at dc), which are modulated by the elements of the sequence L, given by:
L–26, 26 = {1, 1, –1, –1, 1, 1, –1, 1, –1, 1, 1, 1, 1, 1, 1, –1, –1, 1, 1, –1, 1, –1, 1, 1, 1, 1, 0,
1, –1, –1, 1, 1, –1, 1, –1, 1, –1, –1, –1, –1, –1, 1, 1, –1, –1, 1, –1, 1, –1, 1, 1, 1, 1}.
2.10 Pulse shaping and match filter:
The pulse shape of the OFDM symbols preferable to satisfy the orthogonality condition for the multicarrier, two pulses called orthogonal if they don't intersect in time or in frequency domain. It’s not possible to find a pulse limited in the time and the frequency domain at the same time, raised cosine pulse shape used in the OFDM project, with bandwidth B controlled by the rolling factor α by B?T S=1+α.
The raised cosine in the frequency domain represented by:
H f=T
T
2
1+cos
πT
f?
1?β1?β
1+β And in the time domain by: t=sinc t cos πβt T 1? 4β2t2 T2 The roll off factorβ is a measure of the excess bandwidth of the filter, the bandwidth occupied beyond the Nyquist bandwidth, β=2T?f, The following figure 5 and figure 6 show the raised cosine pulse in the frequency and time domain for different roll off factors. Figure 10: Raised cosine for different roll off factor in the frequency domain. Figure 11: Raised cosine for different roll off factor in the time domain. Chapter 3 Implementation 3.1 Transmitter Implementation In this project, Physical Layer of OFDM system based on IEEE 802.11 is implemented .In this chapter, the main parts of OFDM transmitter are illustrated. IEEE mentions following frame structure for the transmission of OFDM signals. The format data transmission includes the OFDM preamble, OFDM header, Data part, tail bits, and pad bits. The Preamble consist of Long Training Sequence (LTS) which is used for the synchronization at the receiver It has very nice auto correlation properties both in time and frequency domain. Figure 11:Autocorrelation in Freq Domain Figure 12: Autocorrelation in Time Domain The header contains the following fields: LENGTH, RATE, a reserved bit, an even parity bit, and the SERVICE field. In terms of modulation, the LENGTH, RATE, reserved bit, and parity bit (with 6 “zero” tail bits appended) constitute a separate single OFDM symbol, denoted SIGNAL, which is transmitted with the most robust combination of BPSK modulation and a coding rate of R = 1/2. The SERVICE field of the PLCP header and the PSDU (with 6 “zero” tail bits and pad b its appended), denoted as DATA, are transmitted at the data rate described in the RATE field and may constitute multiple OFDM symbols. The tail bits in the SIGNAL symbol enable decoding of the RATE and LENGTH fields immediately after the reception of the tail bits. The RATE and LENGTH are required for decoding the DATA part of the packet. Figure 13: Frame Structure for OFDM transmission If seen in time the OFDM transmission will look like the following Figure 14: OFDM Signal Transmission In our Simulation we followed the guideline mentioned by IEEE to generate the OFDM signal except using the short training sequence. WE also added a string of ones to find the phase rotation initially but finally we did the phase rotation as a part of the channel equalization. The implementation of the OFDM transmitter went like below. First we produced the PLCP header field from the RATE, LENGTH, and SERVICE fields of the TXVECTOR by filling the appropriate bit fields. The RATE and LENGTH fields of the PLCP header were encoded by a Convolution code at a rate of R = 1/2, and are subsequently mapped onto a single BPSK encoded OFDM symbol, denoted as the SIGNAL symbol. In order to facilitate a reliable and timely detection of the RATE and LENGTH fields, 6 “zero”tail bits were inserted into the PLCP header. The encoding of the SIGNAL field into an OFDM symbol followed the same steps for Convolution encoding, interleaving, BPSK modulation, pilot insertion, Fourier transform, and prepending a GI (Guard Interval). The contents of the SIGNAL field are not scrambled. Figure 15: Signal Field Calculated from RATE field of the TXVECTOR the number of data bits per OFDM symbol (NDBPS), the coding rate (R), the number of bits in each OFDM subcarrier (NBPSC), and the number of coded bits per OFDM symbol (NCBPS). Table 2: OFDM Parameters for different Modulation Schemes Appended the Data bits to the SERVICE field of the TXVECTOR. Then extended the resulting bit string with “zero” bits (at least 6 bits) so that the resulting length will be a multiple of NDBPS. The resulting bit string constituted the DATA part of the packet. Initiated the scrambler with a pseudorandom non-zero seed, generate a scrambling sequence, and XOR it with the extended string of data bits. In our case the generating polynomial was Figure 17: Scrambler A scrambler is a device that transposes or inverts signals or otherwise encodes a message at the transmitter to make the message unintelligible at a receiver not equipped with an appropriately set descrambling device. Scrambling is accomplished by the addition of components to the original signal or the changing of some important component of the original signal in order to make extraction of the original signal difficult. Examples of the latter might include removing or changing vertical or horizontal sync pulses in television signals; televisions will not be able to display a picture from such a signal. It replaces sequences into other sequences without removing undesirable sequences, and as a result it changes the probability of occurrence of vexatious sequences. Replaced the six scrambled “zero” bits following the “data” with six non-scra mbled “zero” bits. (Those bits return the Convolution encoder to the “zero state” and are denoted as “tail bits.”) Encoded the extended, scrambled data string with a Convolution encoder (R = 1/2). Omit (puncture) some of the encoder output string (chosen according to “puncturing pattern”) to reach the desired “coding rate.” i=N cbps 16 ?k mod 16+ floor k 16 k=0,1,2…N cbps?1 And the second permutation is de?ned by the rule: j= s?floor i + mod i+ N cbps? floor16? i cbps ,s Where N cbps is the number of bits in a single OFDM symbol. Data interleaving spreads data over a variable period of time in order to combat adjacent burst errors .Without data interleaving, many adjacent errors cannot be corrected. With data interleaving, data is transmitted by spacing the content of consecutive packets. Therefore, burst errors are distributed over many data packets, so that the Convolution Decoder has fewer errors to correct in each packet. Divide the resulting coded and interleaved data string into groups of NCBPS bits. For each of the bit groups, convert the bit group into a complex number according to the modulation encoding tables. Table 3:Modulation Encoding table Results for 16-QAM and 64-QAM modulation are shown below Figure 18: 16 QAM Scatter plot Figure 19: 64 QAM Scatter plot Then divided the complex number string into groups of 48 complex numbers. Each such group was associated with one OFDM symbol. In each group, the complex numbers were numbered 0 to 47 and mapped hereafter into OFDM subcarriers numbered –26 to –22, –20 to –8, –6 to –1, 1 to 6, 8 to 20, and 22 to 26. The subcarriers –21, –7, 7, and 21 were skipped and, subsequently, used for inserting pilot subcarriers. The “0” subcarrier, asso ciated with center frequency, was omitted and filled with zero value. Four subcarriers were inserted as pilots into positions –21, –7, 7, and 21. The total number of the subcarriers became 52 (48 + 4). Figure 20: Symbol to FFT Carrier Mapping For each group of subcarriers –26 to 26, converted the subcarriers to time domain using inverse Fourier Transform. Prepend to the Fourier-transformed waveform a circular extension of itself thus forming a GI, and truncate the resulting periodic waveform to a single OFDM symbol length by applying time Domain windowing. In our case the length of the guard interval was ? of the length of 基于FPGA的OFDM系统设计与实现 建立了一个基于FPGA的可实现流水化运行的OFDM系统的硬件平台，包括模拟前端、基于FPGA的OFDM调制器和OFDM 解调器。重点给出了OFDM调制解调器的实现构架，对FPGA实现方法进行了详细的描述，介绍了系统调试方法，并对系统进行了性能评价。 近年来, 随着数字信号处理(DSP) 和超大规模集成电路(VLSI) 技术的发展, 正交频分复用OFDM(Orthogonal Frequency Division Multiplexing)技术的应用有了长足的进步和广阔的发展前景。IEEE802.11a中就将正交频分复用作为物理层的传输技术；欧盟在数字音频广播(DAB)、地面数字视频广播(DVB2T)、高清晰度电视(HDTV)以及2003年4月公布的无线城域网(WMAN)802.16a等研究中都使用了正交频分复用技术作为信道的传输手段。在正交频分复用技术逐渐成熟的今天, 如何降低通信系统的成本, 使之更广泛地应用于数传系统中, 已成为正交频分复用研究的热点。本文基于802.16a协议的原理架构，本着小成本、高效率的设计思想，建立了一个基于FPGA的可实现流水化运行的 OFDM系统的硬件平台，包括模拟前端及OFDM调制器及OFDM 解调器，用来实现OFDM的远距离无线传输系统。 1 模拟前端 模拟前端主要包括发送端DA模块、接收端AD模块和射频模块。 发送端DA模块主要由XILINX公司的FPGA－XC2V1000芯片和数模转换芯片AD9765、滤波器和放大器构成，基带处理调制后数据在控制时钟同步下送入FPGA进行降峰均比等算法的处理，然后经过交织将其送入AD9765进行数模转换并上变频到70MHz，输出的模拟信号再经声表滤波器后放大进入下一级射频模块。发送端DA模块硬件结构框图如图1所示。 OFDM基础理论的数学表达与解析 王海舟 10/10/2016 目录 摘要 (3) 第一章、概述 (4) 第二章、OFDM技术基础理论 (4) 2.1芝诺悖论的哲学来源与泰勒级数 (4) 2.2三角级数和三角函数的正交性 (5) 2.3周期函数的傅里叶级数的表达 (6) 2.4欧拉公式 (8) 2.5非周期连续函数的傅里叶积分变换 (10) 2.6傅里叶变换的时移特性 (11) 2.7单位脉冲函数及其筛选特性 (12) 2.8卷积积分和卷积定理 (14) 2.9奈奎斯特准则和数字滤波初步 (15) 2.10OFDM技术的实现 (17) 第三章、OFDM技术基础理论学习的意义 (18) 摘要 以OFDM技术为基础的LTE通信网络，经过近3年来的高速发展，网络的建设规模方面已经超过GSM网络。4G的Volte语音业务替代2G的步伐也正在加快，而移动数据业务的发展更是一日千里，成为各个运营商竞争的最重要的战场。更何况OFDM技术仍将在未来的5G网络中起着技术基石的作用。 我们知道，2G网络历经了10年以上的发展，大批现场工程师得到了充足的培训，同时又拥有长期的实战经验，因而在网络优化工作中得心应手。相比而言，LTE网络在短时间的发展，致使我们面临短缺具备一定深度基础理论知识的网络优化工程师的情况；尽管工程师能够从多个方面能够取得一些培训，但由于缺少连贯的理论知识对接，这些培训远远不能支持专业的工程师走的更远、走的更深入。面对这样的困境，本人对OFDM技术要点进行理论梳理，从浩瀚的高等数学、工程数学、通信理论的知识海洋中，颉取最简理论线路，创新进行理论关联和演进的串接，不仅令工程师能够夯实最基础的理论，而且用最简捷的数学理论途径，达到深入理解OFDM技术。 关键词： OFDM、泰勒级数、欧拉公式、傅里叶变换、单位脉冲函数、卷积积分、数字滤波。 江阴双威广昊名豪城项目 工程质量控制关键环节控制点 编制单位：祥生建筑安装工程 江阴双威广昊名豪城项目部编制人： 审核人： A结构工程质量控制点 A.1土方开挖工程 1.【控制点】 (1)基底超挖。 (2)基底未保护。 (3)施工开挖顺序不合理。 (4)开挖尺寸不足，边坡过陡，无工作面。 (5)根据地质勘察报告，认真分析地质情况，组织合理、经济的基坑围护方案。 2.【预防措施】 (1)根据结构基础图绘制基坑开挖基底标高图，经审核无误方可使用。 土方开挖过程中，特别是临近基底时，派专业测量人员控制开挖标高。 (2)基坑开挖后尽量减少对基土的扰动，如基础不能及时施工时，应预留30cm土层不挖，待基础施工时再开挖。 (3)开挖时应严格按施工方案规定的顺序进行，先从低处开挖，分层分段，依次进行，形成一定坡度，以利排水。 (4)基底的开挖宽度和坡度，除考虑结构尺寸外，应根据施工实际要求增加工作面宽度。 A.2地下防水 1.【控制点】 (1)防水材料选择，确保选择防水材料技术参数必须符合设计要求。 (2)空鼓。 (3)渗漏。 2.【预防措施】 (1)多方案、多材料的比较，选择一种价格合理，最适合现场实际情况使用的防水材料。 (2)施工时要严格控制基层含水率；卷材铺贴时，要将空气排除彻底，接缝处应认真操作，使其粘结牢固。 对阴阳角、管根等特殊部位，在防水施工前，应做增强处理，可根据具体部位采取有效措施。 (3)卷材末端的收头处理，必须用嵌缝膏或其他密封材料封闭； 防水层施工完成后，要做好成品保护，并及时按设计要求做保护层。 A.3回填土工程 1.【控制点】 (1)未按要求测定土的干密度，回填土的压实系数必须满足设计要求，回填土的选择必须满足设计及施工规的要求。 (2)回填土下沉。 (3)回填土夯压不密实。 (4)管道下部夯填不实。 (5)注意回填分层压实系数及分层厚度。 2.【预防措施】 (1)回填土每层都应测定夯实后的干土密度，检验其密实度，符合设计要求才能铺上层土；未达到设计要求的部位应有处理方法和复验结果。 (2)因虚铺土超过规定厚度或冬期施工时有较大的冻土块，或压实遍数不够，甚至漏压，坑(槽)底有机物或落土等杂物清理不彻底等因素造成回填土下沉，施工中要认真执行规规定，检查发现后及时纠正。 (3)回填时，应在夯压前对于土适当洒水湿润，对土太湿造成的“橡皮土”要挖出换土重填。 (4)回填管沟时，为防止管道中心线位移或损坏管道，应用人工先在管子周围填土夯实，并应从管道两边同时进行，直至管顶0.5m以上，在不损坏管道的情况下，可采用机械回填和压实。 A.4大体积混凝土施工 1.【控制点】 (1)控制裂缝的产生。 (2)混凝土的强度等级及技术参数必须符合设计要求。 (3)混凝土中外加剂必须符合设计及施工验收规要求。 (4)混凝土塔诺度必须满足设计及施工规要求。 (5)注意控制混凝土的浇筑顺序。 交通事故三维模拟演示系统 产品简介 交通事故三维模拟演示系统集成了三维360度全景照相技术、三维虚拟现实动态仿真技术（增强现实技术）为一体，完全满足现在公安系统里现场全景照相、全景三维测量、三维重建、模拟、和分析的应用。是北京金视和科技有限公司集十几年来图形图像和三维仿真领域的尖端科研成果，并结合多年来对公安交通系统的调研数据进行定制化开发的解决方案。 交通事故三维模拟演示系统生成高度逼真的三维场景图片和动画片。把这些全景图片、三维场景、动画片和声音、文字结合，为侦查、技术、指挥人员生成各种三维虚拟案件现场场景的多媒体影音和影像材料。对这些数字化多媒体信息进行分析、演示，并可以在网络服务器上发布、保存、修改案例，其他用户可以通过网络服务器进行查询、观看案例。为案件的侦破、记录、汇报、存档查询，都提供了便利的直观方便。 交通事故三维模拟演示系统是由三维数字化图形软件和360°全自动机器 人拍摄系统组成。是基于图形图像和三维仿真领域的尖端科研成果，并结合多年来对交通事故处理部门的调研数据进行定制化开发的解决方案。 产品特点 交通事故三维模拟演示系统搭载的全景拍摄系统，由高端单反数码相机、精密鱼眼镜头和全自动拍摄云台组成，可以在一分钟内拍摄一组完整的现场全景图片，并以全自动方式进行拼接融合，无须人工干预拼接过程。达到交通事故现场快速全景重建的目的。 在传统的工作流程当中，由于人为、天气等外界因素干扰，事故现场很容易在短时间内遭到破坏和干扰。鉴于事故现场的特殊性，快速、完整、准确的保存事故现场，交通事故现场不可能像刑事案件现场那样长时间保留。交警在记录事故现场时，大多是通过昂贵的单反数码相机，直观的对事故现场进行大量的物证和场景拍摄，在后期分析事故现场时由于照片数量繁多，很难建立起事故现场的形象认知，甚至有可能会漏拍一些关键信息。借助360°全自动机器人拍摄系统将真实交通事故现场的完整的保存下来，可以在撤离交通事故现场后随时在交通事故现场全景图上进行截图、测量和分析。 现场采集物证图像及多场景热点添加在真实的交通事故现场中需要拍摄大量的现场细节图像(车辆痕迹、道路痕迹、现场环境、尸体、现场散落物和遗留物、血迹等)。但大量的现场图片容易让技术人员混淆物证，这对于日后的案情分析来说有重大影响。交通事故三维模拟演示系统的热点添加功能可以将所有采集到的现场细节图像以超级链接方式添加到现场全景图和三维重建现场中，并且可以用鼠标双击放大凸显细节图像，还可以创建文件夹对物证图像进行组织分类、可重命名以及删改细目照片资源，为事故过程分析带来极大的帮助。 课程设计 。 课程设计名称：嵌入式系统课程设计 专业班级： 07级电信1-1 学生姓名：__王红__________ 学号：_____107_____ 指导教师：李国平，陈涛，金广峰，韩琳 课程设计时间：— | 1 需求分析 运用模拟角度调制系统的分析进行频分复用通信系统设计。从OFDM系统的实现模型可以看出，输入已经过调制的复信号经过串／并变换后，进行IDFT或IFFT和并／串变换，然后插入保护间隔，再经过数／模变换后形成OFDM调制后的信号s(t)。该信号经过信道后，接收到的信号r(t)经过模／数变换，去掉保护间隔，以恢复子载波之间的正交性，再经过串／并变换和DFT或FFT后，恢复出OFDM的调制信号，再经过并／串变换后还原出输入符号 2 概要设计 1.简述OFDM通信系统的基本原理 2.简述OFDM的调制和解调方法 3.概述OFDM系统的优点和缺点 4.基于MATLAB的OFDM系统的实现代码和波形 ： 3 运行环境 硬件：Windows XP 软件:MATLAB 4 详细设计 OFDM基本原理 一个完整的OFDM系统原理如图1所示。OFDM的基本思想是将串行数据，并行地调制在多个正交的子载波上，这样可以降低每个子载波的码元速率，增大码元的符号周期，提高系统的抗衰落和干扰能力，同时由于每个子载波的正交性，大大提高了频谱的利用率，所以非常适合移动场合中的高速传输。 在发送端，输入的高比特流通过调制映射产生调制信号，经过串并转换变成N条并行的低速子数据流，每N个并行数据构成一个OFDM符号。插入导频信号后经快速傅里叶反变换(IFFT)对每个OFDM符号的N个数据进行调制，变成时域信号为： [ 式 式1中：m为频域上的离散点；n为时域上的离散点；N为载波数目。为了在接收端有效抑制码间干扰(InterSymbol Interference，ISI)，通常要在每一时域OFDM符号前加上保护间隔(Guard Interval，GI)。加保护间隔后的信号可表示为式，最后信号经并／串变换及D／A转换，由发送天线发送出去。 式 接收端将接收的信号进行处理，完成定时同步和载波同步。经A／D转换，串并转换后的信号可表示为： OFDM技术的基本原理1 OFDM技术的基本原理 在传统的多载波通信系统中，整个系统频带被划分为若干个互相分离的子信道(载波)。载波之间有一定的保护间隔，接收端通过滤波器把各个子信道分离之后接收所需信息。这样虽然可以避免不同信道互相干扰，但却以牺牲频率利用率为代价。而且当子信道数量很大的时候，大量分离各子信道信号的滤波器的设置就成了几乎不可能的事情。 上个世纪中期，人们提出了频带混叠的多载波通信方案，选择相互之间正交的载波频率作子载波，也就是我们所说的OFDM。这种“正交”表示的是载波频率间精确的数学关系。按照这种设想，OFDM既能充分利用信道带宽，也可以避免使用高速均衡和抗突发噪声差错。OFDM是一种特殊的多载波通信方案，单个用户的信息流被串/并变换为多个低速率码流，每个码流都用一个子载波发送。OFDM不用带通滤波器来分隔子载波，而是通过快速傅立叶变换(FFT)来选用那些即便混叠也能够保持正交的波形。 OFDM是一种无线环境下的高速传输技术。无线信道的频率响应曲线大多是非平坦的，而OFDM技术的主要思想就是在频域内将给定信道分成许多正交子信道，在每个子信道上使用一个子载波进行调制，并且各子载波并行传输。这样，尽管总的信道是非平坦的，具有频率选择性，但是每个子信道是相对平坦的，在每个子信道上进行的是窄带传输，信号带宽小于信道的相应带宽，因此就可以大大消除信号波形间的干扰。由于在OFDM系统中各个子信道的载波相互正交，它们的频谱是相互重叠的，这样不但减小了子载波间的相互干扰，同时又提高了频谱利用率。 OFDM技术的推出其实是为了提高载波的频谱利用率，或者是为了改进对多载波的调制，它的特点是各子载波相互正交，使扩频调制后的频谱可以相互重叠，从而 关键工序质量控制系统使用说明 一、关键工序质量控制系统的使用范围： 目前，关键工序质量控制系统仅包括铝合金门窗安装和卫生间防水工程。 二、关键工序质量控制系统的内容： 关键工序质量控制系统作为加强关键工序过程控制的管理工具，要求项目工程管理部门对关键工序施工过程中的各质量控制点进行检查、验收、记录、存档。同时将系统记录的电子文档上传集团总部管理平台。 关键工序质量控制系统由以下三部分组成： 1、关键工序质量控制平面图； 2、施工验收记录表； 3、过程监控照片记录。 三、关键工序质量控制平面图的使用说明：（详附件一中的二层铝合金门窗工程质量控制平面图.doc） 1、关键工序质量控制平面图由甲方代表在施工前准备， 质量控制平面图应包含检查验收所需的必要内容，如验收部位的楼层平面位置、验收对象的指示编号，验收对象必要标注的示意或说明等； 2、关键工序质量控制平面图要求链接对应检查的验收记录附表； 3、关键工序质量控制平面图制作时应考虑施工验收的程序及计划，尽量对验收对象按类别、户型、进度、部位进行合并以便于质量控制资料的整理。 四、施工验收记录表使用说明：（详附件一记录附表中的施工验收表-二层铝合金门窗 1 表.doc） 1、施工验收记录表格的具体填写方式见附页的填表说明； 2、表格的验收内容可根据工程的具体设计要求和总部颁发的施工验收标准进行调整，以求符合项目实际施工情况，但表体格式及验收标准不得改动； 3、验收记录表格应与质量控制平面图及相关照片记录链接对应； 五、过程监控照片记录文档的拍摄、编辑说明：（详附件一 中的监控照片-铝合金窗 1 .doc） 1、过程监控照片记录文档应与被验收的对象对应，并与施工验收表格对应链接； 2、过程监控照片的内容应能够如实反映该道工序质量控制点隐蔽之前情况，如卫生间防水工程主要反映面层施工之前各层做法施工过程的情况，铝合金门窗工程主要反映洞口抹灰收口前的隐蔽情况； 3、过程监控照片由施工方质量负责人现场拍摄，拍摄时要求在图像内表明验收的时间、部位及对象名称、编号，并有监理工程师及甲方代表在场； 4、过程监控照片记录文档由一张或多张过程监控照片嵌入组成，照片的格式要求为jpg格式，分辨率为640×480像素。 六、关键工序质量控制系统资料的整理与上报： 1、关键质量控制系统的验收记录及监控照片等原始资料由甲方代表、监理工程师各整理一份，工程部资料员应对关键工序质量控制系统资料（纸面文档和电子文档）存档备查。 2、关键质量控制系统资料的电子文档由甲方代表随原始资料的积累及时整理、编辑而成，质量控制系统资料的电子文档 基于matlab的ofdm系统设计与仿真 摘要 OFDM即正交频分复用技术，实际上是多载波调制中的一种。其主要思想是将信道分成若干正交子信道，将高速数据信号转换成并行的低速子数据流，调制到相互正交且重叠的多个子载波上同时传输。该技术的应用大幅度提高无线通信系统的信道容量和传输速率，并能有效地抵抗多径衰落、抑制干扰和窄带噪声，如此良好的性能从而引起了通信界的广泛关注。 本文设计了一个基于IFFT/FFT算法与802.11a标准的OFDM系统，并在计算机上进行了仿真和结果分析。重点在OFDM系统设计与仿真，在这部分详细介绍了系统各个环节所使用的技术对系统性能的影响。在仿真过程中对OFDM信号使用QPSK调制，并在AWGN信道下传输，最后解调后得出误码率。整个过程都是在MATLAB环境下仿真实现，对ODFM系统的仿真结果及性能进行分析，通过仿真得到信噪比与误码率之间的关系，为该系统的具体实现提供了大量有用数据。 第一章 ODMF 系统基本原理 1.1多载波传输系统 多载波传输通过把数据流分解为若干个子比特流，这样每个子数据流将具有较低的比特速率。用这样的低比特率形成的低速率多状态符号去调制相应的子载波，构成了多个低速率符号并行发送的传输系统。在单载波系统中，一次衰落或者干扰就会导致整个链路失效，但是在多载波系统中，某一时刻只会有少部分的子信道会受到衰落或者干扰的影响。图1－1中给出了多载波系统的基本结构示意图。 图1-1多载波系统的基本结构 多载波传输技术有许多种提法，比如正交频分复用(OFDM)、离散多音调制(DMT)和多载波调制(MCM)，这3种方法在一般情况下可视为一样，但是在OFDM 中，各子载波必须保持相互正交，而在MCM 则不一定。 1.2正交频分复用 OFDM 就是在FDM 的原理的基础上，子载波集采用两两正交的正弦或余弦函数集。函数集{t n ωcos }， {t m ωsin } (n,m=0,1,2…)的正交性是指在区间(T t t +00,)内有正弦函数同理：)0()()(2/0cos *cos 00===≠?? ???=? +m n m n m n T T tdt m t n T t t ωω 其中ωπ2=T （1-1） OFDM 的基本原理及其简单应用 摘要：本文主要介绍OFDM 的一些基本原理，并对OFDM 的一些优缺点进行了说明。正交频分复用（OFDM ）是一种特殊的多载波数字调制技术，OFDM 技术不像常规的单载波技术，而是在经过特别计算的正交频率上同时发送多路高速信号。介绍了OFDM 的基本原理的同时展望了OFDM 标准化和在第四代移动通信系统的应用。 关键词：OFDM ，DFT/IDFT ，多载波调制，数字通信 中图分类号：TN911 文献标致码：A Basic Principles and Simple Applications Of OFDM （Xi’an university of science and technology Communication and Information Systems Institute shanxi xi ’an 710054） Abstract ：In this article ，the principle of OFDM are introduced and OFDM are described some of the advantages and disadvantages. OFDM(orthogonal frequency division multiplexing) is a special digital modulation technology of multi-carriers. Unlike normal single carrier technology , OFDM can transmit a number of data streams simultaneously through its sub- carriers which are orthogonal. In the end, highlighted the standardization of OFDM and its applications in 4G mobile communication system. Key W ords ：OFDM ，DFT/IDFT ，Multi-carrier modulation ，Digital communications 0.引言 随着移动通信和数据通信的飞速发展，移动用户对业务种类和通信速率的要求不断提高，正交频分复用（OFDM ）具有高的频谱利用率、良好的抗多径干扰能力和抗短时间突发噪声（称为脉冲噪声）的能力，它可以增加系统容量，同时能更好地满足多媒体通信的要求。OFDM 是多载波调制（MCM ）或离散多音频（DMT ）的一种特殊形式，是一类多载波并行调制的体制，一种带宽有效性较高的调制技术，并可以对抗时延扩展多径和脉冲噪声等信道干扰。它的一些主要特点是： （1）为了提高频率利用率和增大传输速率，各路子载波的已调信号频谱有部分重叠。 （2）各路已调信号是严格正交的，以便接收端能完全的分离各路信号。 （3）每路子载波的调制是多进制调制。 （4）每路子载波的调制制度可以不同，根据各个子载波处信道特性的优劣不同采用不同的体制。 1.OFDM 的基本原理 1.1 多载波的基本原理 多载波就是把传输的宽带分成许多窄带子载波来并行传输，多载波可以在有限的无线传播带宽中获得更高的传输速率。在单载波体制的情况下，码元持续时间T 很短，但占用带宽B 很大，由于信道特性不理想，产生码间串扰。采用多载波后码元持续时间S T N T ，码间串扰将得到改善。 内部控制系统的关键控制点 为了防止资产流失，一个公司可采取的控制措施有很多，下面所列的是在大多数公司里最常采用的一些措施。在资产流失机率比较大的情况下，也可以在这些措施的基础上增加一些其他的相关措施，反之亦然。这些控制措施包括下列几方面内容。 (1）现金 通常现金交易被认为是最需要进行控制的环节。因此，有关现金控制在有些企业甚至显得有些过度。尽管下面介绍了许多的现金控制措施，但我们在实际执行的时候，应充分考虑成本与收益的关系，将多种措施综合起来使用，效果会更好。 ◎将登记入账的支票号码与实际的支票号码核对。电脑打印出的支票清单应与实际使用的支票完全一致。如果不一致，就说明可能有人私自娜用支票。在采用激光打印支票的情况下，这种违法行为更容易发生。因为采用激光打印，发票是分别存放的，这比使用连续编号的支票簿更容易被偷盗。 ◎对小额现金进行现场盘点。使用各种收据或借条来抵库，是小额现金常见的舞弊方式。因此，不定期进行审计，盘点现金余额，就可能发现这些舞弊行为。 ◎支票簿的控制。支票簿不应和纸笔一起放在柜子里，因为任何人都有可能从支票簿里偷走一张支票，使用伪造的签章就可以将公司的资金支走。因此，支票簿应该放在保险柜里，未经授权任何人不得接近动用。。公章的控制。如果任何人都有机会接触到公章，那么不仅公司的支票可能被冒支，偷盗者还有可能利用公章以公司的名义签订各种合同。因此，公司的公章也应保存在公司的保险柜里。 ◎公司邮政收发部门应建立一套收发记录清单。如果收到客户的支票后，在登记银行存款日记账之前，会计部门成员有可能会接触到这些支票。邮政收发部门单独建立一套收发日记账，以备日后和会计部门的存款日记账相核对。 南华大学电气工程学院 《通信原理课程设计》 设计题目： OFDM调制解调系统的设计 专业：电子信息工程 学生姓名: 谭晓倩学号: 20114470203 起迄日期: 2014年5月24日—2014年6月6日指导教师:李圣 系主任：陈忠泽 《通信原理课程设计》任务书 III IV 附件二： 《通信原理课程设计》设计说明书格式 一、纸张和页面要求 A4纸打印；页边距要求如下：页边距上下各为2.5 厘米，左右边距各为2.5厘米；行间距取固定值（设置值为20磅）；字符间距为默认值（缩放100%，间距：标准）。 二、说明书装订页码顺序 (1)任务书 (2)论文正文：包括中英文摘要、目录、绪论、方案设计、硬软件设计调试（仿真过程设计及调试）、分析结论 (3)参考文献（5篇以上），(4)附录 三、课程设计说明书撰写格式 见范例 设计任务及指标(黑体四号) ☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆(首行缩进两个字，宋体小四号) 1☆☆☆☆(黑体四号) 正文……(首行缩进两个字，宋体小四号) 1.1（空一格）☆☆☆☆☆☆(黑体小四号) 正文……(首行缩进两个字，宋体小四号) 1.2 ☆☆☆☆☆☆、☆☆☆ 正文……(首行缩进两个字，宋体小四号) 2 ☆☆☆☆☆☆ (黑体四号) 正文……(首行缩进两个字，宋体小四号) 2.1 ☆☆☆☆、☆☆☆☆☆☆，☆☆☆(黑体小四号) 正文……(首行缩进两个字，宋体小四号) 2.1.1☆☆☆，☆☆☆☆☆，☆☆☆☆ (楷体小四号) 正文……(首行缩进两个字，宋体小四号) （1）…… ①…… V 5结论(黑体四号) ☆☆☆☆☆☆(首行缩进两个字，宋体小四号) 图1. 工作波形示意图(图题，居中，宋体五号) 参考文献(黑体四号、顶格) 参考文献要另起一页，一律放在正文后，不得放在各章之后。只列出作者直接阅读过或在正文中被引用过的文献资料，作者只写到第三位，余者写“等”，英文作者超过3人写“et al”。 几种主要参考文献著录表的格式为： ⑴专(译)著：[序号]著者.书名（译者）[M].出版地：出版者，出版年：起~止页码. ⑵期刊：[序号]著者.篇名[J].刊名，年，卷号（期号）：起~止页码. ⑶论文集：[序号]著者.篇名[A]编者.论文集名[C] .出版地：出版者，出版者. 出版年：起~止页码. ⑷学位论文：[序号]著者.题名[D] .保存地：保存单位，授予年. ⑸专利文献：专利所有者.专利题名[P] .专利国别：专利号，出版日期. ⑹标准文献：[序号]标准代号标准顺序号—发布年，标准名称[S] . ⑺报纸：责任者.文献题名[N].报纸名，年—月—日（版次）. 附录（居中,黑体四号） ☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆☆（首行缩进两个字，宋体小四号。另起一页。附录的有无根据说明书（设计）情况而定，内容一般包括正文内不便列出的冗长公式推导、符号说明（含缩写）、计算机程序、整体原理图、印制电路板图等。) VI OFDM 的基本原理 杜岩 （山东大学信息科学与工程学院济南 250100） 1. 引言 现代社会对通信的依赖和要求越来越高，于是设计和开发效率更高的通信系统就成了通信工程界不断追求的目标。通信系统的效率，说到底就是频谱利用率和功率利用率。特别是在无线通信的情况下，对这两个指标的要求往往更高，尤其是频谱利用率。由于空间可用频谱资源是有限的，而无线应用却越来越多，使得无线频谱的使用受到各国政府的严格管理并统一规划。于是，各种各样的具有较高频谱效率的通信技术不断被开发出来，OFDM （Orthogonal Frequency Division Multiplexing）是目前已知的频谱利用率最高的一种通信系统，它将数字调制、数字信号处理、多载波传输等技术有机结合在一起，使得它在系统的频谱利用率、功率利用率、系统复杂性方面综合起来有很强的竞争力，是支持未来移动通信特别是移动多媒体通信的主要技术之一。 OFDM是一种多载波传输技术，N个子载波把整个信道分割成N个子信道，N个子信道并行传输信息。OFDM系统有许多非常引人注目的优点。第一，OFDM具有非常高的频谱利用率。普通的FDM系统为了分离开各子信道的信号，需要在相邻的信道间设置一定的保护间隔（频带），以便接收端能用带通滤波器分离出相应子信道的信号，造成了频谱资源的浪费。OFDM系统各子信道间不但没有保护频带，而且相邻信道间信号的频谱的主瓣还相互重叠（见图1.5），但各子信道信号的频谱在频域上是相互正交的，各子载波在时域上是正交的，OFDM系统的各子信道信号的分离（解调）是靠这种正交性来完成的。另外，OFDM 的个子信道上还可以采用多进制调制（如频谱效率很高的QAM），进一步提高了OFDM系统的频谱效率。第二，实现比较简单。当子信道上采用QAM或MPSK调制方式时，调制过程可以用IFFT完成，解调过程可以用FFT完成，既不用多组振荡源，又不用带通滤波器组分离信号。第三，抗多径干扰能力强，抗衰落能力强。由于一般的OFDM系统均采用循环前缀（Cyclic Prefix，CP）方式，使得它在一定条件下可以完全消除信号的多径传播造成的码间干扰，完全消除多径传播对载波间正交性的破坏，因此OFDM系统具有很好的抗多径干扰能力。OFDM的子载波把整个信道划分成许多窄信道，尽管整个信道是有可能是极不平坦的衰落信道，但在各子信道上的衰落却是近似平坦的（见图1.6），这使得OFDM系统子信道的均衡特别简单，往往只需一个抽头的均衡器即可。 当然，与单载波系统比，OFDM也有一些困难问题需要解决。这些问题主要是：第一，同步问题。理论分析和实践都表明，OFDM系统对同步系统的精度要求更高，大的同步误差不仅造成输出信噪比的下降，还会破坏子载波间的正交性，造成载波间干扰，从而大大影响系统的性能，甚至使系统无法正常工作。第二，OFDM信号的峰值平均功率比（Peak-to-Average Power Ratio，PAPR）往往很大，使它对放大器的线性范围要求大，同时也降低了放大器的效率。OFDM在未来通信系统中的应用，特别是在未来移动多媒体通信中的应用，将取决于上述问题的解决程度。 OFDM技术已经或正在获得一些应用。例如，在广播应用中欧洲的ETSI（European Telecommunication Standard Institute，欧洲电信标准学会）已经制定了采用OFDM技术的数 移动通信系统课程设计报告 OFDM系统仿真 —— 目录 移动通信系统课程设计报告 (1) (一)题目要求： (2) (二)相关原理： (2) 1)OFDM： (2) 2)QPSK调制： (3) 3)导频与均衡： (3) 4)循环前缀： (3) 5)分组交织： (4) (三)基本思路： (4) (四)结果： (10) 1)软解码与硬解码情况下不同信噪比的误码率： (10) 2)不同信噪比下译码相位图： (11) (五)总结体会： (12) (六)分工合作： (13) (七)程序代码： (13) (一)题目要求： 1)OFDM128路传输； 2)QPSK调制 3)AWGN信道 4)3径或4径瑞利衰落信道 (二)相关原理： 1)OFDM： 将信道分成若干正交子信道，将高速数据信号转换成并行的低速子数据流，调制到在每个子信道上进行传输。正交信号 可以通过在接收端采用相关技术来分开，这样可以减少子信道 之间的相互干扰(ISI) 。每个子信道上的信号带宽小于信道的 相关带宽，因此每个子信道上可以看成平坦性衰落，从而可以 消除码间串扰，而且由于每个子信道的带宽仅仅是原信道带宽 的一小部分，信道均衡变得相对容易。 2) QPSK 调制： 将每两个相连比特组在一起形成双比特码元，它的四种状态用4个不同的相位表示； 3) 导频与均衡： 在OFDM 信息序列中插入已知的导频序列()x n ，通过信道后将其提取得()y n ，做频域除法得传输函数[][]z =[] Y z H X z ，再通过线性插值后得到每个信道频率响应，均衡滤波传输函数[]1E [] z H z =； 4) 循环前缀： 循环前缀(Cyclic Prefix, CP)是将OFDM 符号尾部的信号搬移到头部构成的。用来消去码间干扰，通常取长度g T τ≥（τ为信道冲激响应持续时间） 质量控制重点及难点 桩基成孔质量控制；预应力施工质量控制；保护层厚度控制；砼外观质量和桥面平整度控制措施。 1.1桩基成孔质量保证措施 本项目地质条件较差，水文地质情况复杂，施工工艺多样，有冲击钻、旋挖钻等。如何确保成桩质量的同时提高施工效率，是决定工程施工成败的关键。 （1）、避免斜孔、塌孔 ①钻机底座牢固可靠，钻机不得产生水平位移和局部沉降； ②钢护筒埋设准确，且用建议平台进行固定防止倾斜。钻进过程中及时复核垂直度，发现倾斜及时处理； ③钢护筒跟进及时，冲击钻锤头始终保持不超出护筒。 （2）、预防糊钻、卡钻、掉钻 冲击钻要加强钻头、钢丝绳、钢丝绳与卡扣连接处、钢丝绳接头处的磨损、锈蚀等情况的检查。现场配备冲击空心锤，一旦卡钻立即处理。 （3）、意外坠落物预防及处理 现场准备充足打捞设备，如果不慎发生掉钻等掉落物事故可根据实际情况迅速采取措施，以防沉淀埋住掉落物增加打捞难度，根据以往积累的施工经验，在现场准备2台8t冲机、8t电磁铁、300t起重架、300t液压千斤顶等整套吸吊设备。若掉钻可采用多向打捞钩打捞， 速度快，成功率高；若掉落钢板等小块铁件可用电磁铁打捞，若埋钻可用300t起重架反顶起吊。打捞过程中注意保孔，如置换高性能优质泥浆、维持桩孔外水头高差。 （4）、防止沉渣过厚 ①成孔钻进距孔底标高差50cm左右时，量准尺寸，严格进尺速度，到位后即进行清孔。 ②采用反循环清孔，泥浆处理器持续循环除砂，降低含砂率。（5）、预防成孔灌注砼时堵管、断桩现象出现 ①专人负责跟踪原材料验收及使用，规拌料配比及半成品验收，保证混凝土级配，加快灌注时间。若连续灌注中，出现堵管，可在保证导管埋深2m以上的前提下，上下吊插导管振捣，可利于砼下溜。 ②断桩预防措施：导管用前，需通过试压、拉伸试验，并舍弃、替换残久或壁厚较薄的导管。灌注中对导管埋深仔细记录，并复核已浇砼量，返算砼面标高，看与记录是否相符，始终保持导管埋深在2m以上。 1.2预应力施工质量控制 一、波纹管方面 通病表现之一： 波纹管材质低劣，成品质量不合格，表现为其整体强度、刚度不符合标准，螺旋卷压接缝咬合不牢固、不严密。管材厚度、硬度不符合标准。预防及治理(补救)措施： 严把材料质量关，采用产品质量好的厂家的产品，必须要有出厂 模拟演练与3D培训系统方案 煤科集团沈阳研究院有限公司 2014/2/28 一、模拟演练技术资料 DMX-135A仿真模拟与演练评价系统说明 1.系统型号 DMX-135 A 通道长度 多功能模拟训练系 统 2．系统简介 “3D-VR煤矿事故仿真和安全培训 演练系统”是我院与澳大利亚新南威尔 士大学合作开发的一套应用于煤矿安全培训的大型沉浸式仿真演练系统。我院通过借助UNSW(新南威尔士大学)的世界领先技术及软硬件平台，开发出符合我国国情的煤矿安全培训模块。本系统可由危险识别、灾害模拟、自救逃生、救护救援等多个安全培训模块组成，本系统模块是按照国家煤矿安全规程和新编煤矿安全技术培训教材编制 而成。采用虚拟现实（VR）技术、360度环屏投影播放技术、12.1环绕立体声技术、3D电影技术、计算机网络协同处理技术为煤矿工人安全和技能培训提供一流的“沉浸式”培训环境，将安全培训装备和质量提升到一个崭新的水平。 通过使用本系统，创建一个逼真的虚拟现实世界，使学员在体验各种真实场景同时，学习各模块相关操作知识，认识灾害的发生、发展过程及危害，并通过问答式的交互学习，快速掌握每个模块的操作步骤及要点。从而提高矿工的整体技术水平、思想素质、及各种突发状况的应对能力，从而使煤矿整体操作能力及防范意识得到提高，真正进入规范化管理阶段，降低各种事故发生的概率。 DMX-135A多功能模拟训练系统通道总长135米，分三层，旨在通 过仿真救灾现场的严峻条件，人为设置测试科目，使训练人员背负呼吸器，在黑暗、噪音、浓烟、高温的模拟条件下，按照预先设定的工作程序，完成正确穿越各种障碍，并按规定动作完成抢险、伤员营救等工作。它可以测量出训练人员最大身体承受能力和心理承受能力，评定受训人员能否正确使用呼吸器及抢险救援任务等的完成情况。 最 额定电压：380V，允许偏差±10%； 谐波：不大于5%； 频率：50Hz，允许偏差±5%； 3．3 系统组成 系统由以下部分组成: O F D M系统设计及基带系 统仿真 Revised by Jack on December 14,2020 OFDM原理与应用课程设计 OFDM系统设计及基带系统仿真 学号： 专业：信息与通信工程 学生姓名：段京京 任课教师：张薇副教授 2016年4月 第1章绪论 引言 计算机技术、Internet网络的发展与普及改变了人类生活方式，这是人类科技的一次革命性的进步。随着人们对信息量的需求越来越多，无线移动通信进入了一个快速发展时期。进入21世纪以来，国内外移动通信技术有着更快速的发展，特别是无线通信网络和Internet的结合，使网络资源发挥了更大的作用，更加促进了Internet的发展和无线移动网络的完善，人们的生活方式更加便捷和多样化，世界发展更快、更加精彩、更加辉煌。无线移动通信技术迎来了又一次伟大的变革。其中，正交频分复用（OFDM）技术是其关键技术。 在现代移动通信系统的无线信道中，随着传输数据率的提高，多径衰落和由之引起的码间串扰会严重影响系统性能。克服这种影响的一种方法是采用信道均衡技术，但是随着数据传输速率的提高，其代价可能变得无法接受。正交频分复用（OFDM）传输技术提供了让数据以较高的速率在较大延迟的信道上传输的另一种途径。OFDM技术是一种多载波调制技术，它将串行高速信息数据流变换成为若干路并行低速数据流，每路低速数据流被调制在彼此正交的子载波上构成发送信号。由于OFDM具有较高的频谱利用率及抗多径干扰能力强的优点，且能够通过IFFT/FFT等高效算法实现，因此目前它已成为应用最为广泛的多载波调制方式。 OFDM系统的发展 上个世纪70年代，Weinstein和Ebert等人应用离散傅里叶变换和快速傅里叶变换创造了一个完整的多载波传输系统，叫做正交频分复用（OFDM）系统。 摘要 随着我国汽车保有量的大幅度上升，高新技术产品和装置在汽车上的不断引入和普及与汽车维修业高素质从业人员不足之间的矛盾显得日益突出。发动机电控技术是汽车专业的一门核心技术，理论性与实践性很强。开发发动机电控系统模拟教学实验台，能便于学生认识和学习发动机电控系统的工作原理。发动机电控系统模拟实验台可以广泛应用于科研、教学等方面，通过对发动机工作过程的模拟、显示，可以提高汽车从业人员的知识掌握水平及实际操作能力，增强对汽车电控系统的认识和掌握，为学生学习和掌握发动机电控系统的工作原理提供帮助。 本设计是以教学展示为前提，主要介绍了发动机电控系统的控制内容、发展简史与前景、组成与工作原理，并在此基础上设计电控系统模拟实验台的相关内容。主要包括实验台架的设计、电控系统的零部件布置、电动机转速控制等内容。这个教学实验台能够演示发动机的起动和正常工作过程，并且设置了测量点。通过完成对实验台的设计，可以深刻理解和掌握发动机电控系统的组成和工作原理、机械设计与机械制图的相关知识，最终设计出一款及机械、电控于一体化的产品。 关键词：发动机电控系统；电动机控制；实验台；单片机；PWM ABSTRACT With significant increase of the number of automobile，the contradiction between high-tech products and devices, the constant introduction in the car and popularization of high-quality vehicle maintenance trade and lack of practitioners become increasingly prominent. Engine electronic control technology is the theoretical and practical of a core technology and theoretical and practical very strong. Electronic control system simulation teaching test can make students to understand and learn the engine electronic control system works easy. Engine electronic control simulation system can be widely used in scientific research, teaching, etc. By displaying of the course of the engine simulation process, it can be employed to improve motor vehicle mastery level of knowledge and practical ability, and enhance the automotive electronic control system of understanding and mastering, for the students to learn and master the engine control system works to help. The design is based on the premise of teaching show, this paper introduces control content system of the engine electronic control, brief history of and prospects for development, composition and working principle and design on the basis of electronic control system simulation test-bed of related content. It mainly includes the design of bench testing, electrical control system layout of components, such as motor speed control. The teaching experiment platform to demonstrate the engine start and normal work processes, and set the measurement points. The design of the test-bed by accomplish can help learns to understand and master the engine electronic control system components, a final design and mechanical and electrical control products in the integration. Key words: Engine Electrical Control System；Motor Control；Test-bed；microcontroller；PWM基于FPGA的OFDM系统设计与实现
OFDM基础理论的数学表达和解析(end)汇总
工程高质量控制系统关键环节点
交通事故三维模拟演示系统
OFDM系统设计及其Matlab实现
OFDM技术的基本原理1
关键工序质量控制系统使用说明
无线通信原理 基于matlab的ofdm系统设计与仿真..
OFDM的基本原理和简单应用
内部控制系统的关键控制点
OFDM调制解调系统的设计
OFDM的基本原理
移动通信系统课设_OFDM系统仿真设计
高质量控制系统重点及难点
模拟演练与培训系统方案
OFDM系统设计及基带系统仿真
汽油机电控系统模拟教学演示台设计
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